The invention relates to a boron suboxide composite material.
The development of synthetic ultrahard materials which have hardness values approaching or even exceeding that of diamond has been of great interest to material scientists. With a Vickers hardness of between 70 to 100 GPa, diamond is the hardest material known, followed by cubic boron nitride (HV˜60 GPa) and boron suboxide, herein referred to as B6O. Hardness values of 53 GPa and 45 GPa have been determined at 0.49 N and 0.98 N load respectively for B6O single crystals, which are similar to those of cubic boron nitride [9].
It is known that B6O may also be non-stoichiometric i.e. exist as B6O1-x (where x is in the range 0 to 0.3). Such non-stoichiometric forms are included in the term B6O. The strong covalent bonds and short interatomic bond length of these materials contribute to the exceptional physical and chemical properties such as great hardness, low mass density, high thermal conductivity, high chemical inertness and excellent wear resistance [1, 2]. In U.S. Pat. No. 5,330,937 to Ellison-Hayashi et al the formation of boron suboxide powders of nominal composition B3O, B4O, B6O, B7O, B8O, B12O, B15O and B18O was reported. Potential industrial applications have been discussed by Kurisuchiyan et al (Japan Patent No. 7,034,063) and Ellison-Hayashi et al (U.S. Pat. No. 5,456,735) and include use in grinding wheels, abrasives and cutting tools.
Several techniques have been employed for producing boron suboxide and include such procedures as reacting elemental boron (B) with boron oxide (B2O3) under suitably high pressure and high temperature conditions [1]. In U.S. Pat. No. 3,660,031 to Holcombe Jr. et al other methods of producing boron suboxides such as reducing boron oxide (B2O3) with magnesium, or by reducing zinc oxide with elemental boron are mentioned. With each of these known procedures however, there are drawbacks which retard the usefulness of the material in industry. For example, the reduction of B2O3 with magnesium produces a solid solution of magnesium and magnesium boride contaminants in the suboxide, while the reduction of magnesium oxide with boron produces only a relatively small yield of boron suboxide and is very inefficient. Holcombe Jr. et al (U.S. Pat. No. 3,660,031) produced B7O by reducing zinc oxide with elemental boron at temperatures of between 1200° C. to 1500° C. A hardness value of 38.2 GPa under 100 g load and density of 2.6 g.cm−3 is reported for this material. The fracture toughness for this material is not discussed, because only grid and not dense materials were produced.
Petrak et al [3] investigated the mechanical and chemical properties of hot-pressed B6O and reported micro-hardness values as high as 34-38 GPa. Ellison-Hayashi et al (U.S. Pat. No. 5,330,937) produced B6O with a magnesium addition (approximately 6%) which yielded average KHN100 values of 34 GPa to 36 GPa.
Efforts have been made to enhance the mechanical properties of B6O, especially its fracture toughness, by forming B6O composites with other hard materials such as diamond [4], boron carbide [5], and cBN [6]. The diamond and cBN-containing composites were made under extremely high temperature and pressure conditions. The intention was to form pseudo-binary composite systems, stronger at the grain boundaries than those of pure B6O. Even though high hardness values were recorded for the composites (HV˜46 GPa), again, fracture toughness values did not exceed 1.8 MPa·m0.5. The best value here was obtained with B6O-cBN composites.
Shabalala et al (WO 2007/029102 and [7]) produced B6O composites with aluminium compounds which resulted in an aluminium borate phase at the grain boundary. A fracture toughness of about 3.5 MPa·m0.5 with a corresponding hardness of 29.3 GPa was obtained. The aluminium phases present in the composite are soft and although they may improve the fracture toughness of the resulting composite, they do not contribute to the overall hardness of the composite. Moreover, in addition to a crystalline aluminium borate, a boron oxide rich, chemically unstable amorphous phase and microporosity was formed, further resulting in reduced hardness [10, 11].